1. Field of the Invention
The present invention relates to surface acoustic wave filters suitable for use in mobile communication apparatuses such as mobile phones. More particularly, the present invention relates to wiring structures of longitudinally-coupled resonator-type surface acoustic wave filters.
2. Description of the Related Art
As a conventional band pass filter used in the RF stage of a mobile communication apparatus such as a mobile phone, a surface acoustic wave filter is well known. In general, capabilities required in such a band pass filter include characteristics of low loss, high attenuation, a broad band, and other known characteristics. Thus, in order to improve these characteristics of the surface acoustic wave filter, there have been many inventions developed in the past.
For example, in order to obtain high attenuation in a longitudinally-coupled resonator-type surface acoustic wave filter, a method is known in which a surface acoustic wave filter is constituted by longitudinally connecting a plurality of longitudinally-coupled resonator-type surface acoustic wave elements, as described in Japanese Unexamined Patent Application Publication No. 5-335881. In this method, since the plurality of surface acoustic wave elements are longitudinally connected to each other, there is a disadvantage that insertion loss within the pass band increases. On the other hand, high attenuation outside the pass band can be obtained.
However, when constituting a filter having a broad pass bandwidth by longitudinally connecting the longitudinally-coupled resonator-type surface acoustic wave elements as mentioned above, waves within the pass band and the voltage standing wave ratio (VSWR) both become large. Next, the cause of the problem will be explained with reference to FIG. 6.
FIG. 6 is a plan view of a conventional surface acoustic wave filter 100 constructed by longitudinally connecting two longitudinally-coupled resonator-type surface acoustic wave elements. In this figure, the reference numeral 138 denotes a piezoelectric substrate made of LiTaO3. The reference numerals 101 and 102 denote longitudinally-coupled resonator-type surface acoustic wave elements arranged on the piezoelectric substrate 138. The surface acoustic wave element 101 is composed of an IDT (interdigital transducer) 103, two IDTs 104 and 105 located on each side of the IDT 103, and two reflectors 106 and 107 disposed on each side of the arrangement of the IDTs 104, 103, and 105. In the same manner, the surface acoustic wave element 102 is composed of IDTs 108, 109, and 110, with reflectors 111 and 112. The two surface acoustic wave elements 101 and 102 are longitudinally connected to each other to constitute the surface acoustic wave filter 100. The filter 100 is electrically connected to bonding pads 123 to 128 on a package 137 via bonding wires 129 to 136. The bonding pad 124 is used as an input terminal, the bonding pad 127 is used as an output terminal, and the bonding pads 123, 125, 126, and 128 are used as ground terminals. In addition, electrode pads 113 to 120 are provided on the piezoelectric substrate 138 so as to correspond to the bonding pads 123 to 128 disposed on the package 137. The electrode pad 114 is used as an input terminal, the electrode pad 119 is used as an output terminal, and the electrode pads 113, 115, 116, 117, 118, and 120 are used as ground terminals. Furthermore, in order to longitudinally connect the surface acoustic wave elements 101 and 102, an electrode pattern 121 for connecting the IDTs 104 and 109 and an electrode pattern 122 for connecting the IDTs 105 and 110 are disposed on the piezoelectric substrate 138.
In the surface acoustic wave filter 100 having the above-described arrangement, parallel capacitances are generated between the electrode pad 116 as a ground terminal and the electrode patterns 121 and 122 and between the electrode pad 117 as a ground terminal and the electrode patterns 121 and 122, respectively. When the parallel capacitances are generated between the ground-terminal pads and the electrode patterns through which a passing signal is transmitted, the impedance of each element viewed from the interstage junction tends to be capacitive.
Primarily, in a surface acoustic wave filter having a broad pass bandwidth, the impedance tends to be capacitive. As shown in the conventional filter in FIG. 6, in the structure in which the parallel capacitances are generated at the interstage junction of the longitudinally connected surface acoustic wave elements, the impedance tends to be more capacitive. As a result, waves within the pass band and the VSWR both become larger, resulting in the filter characteristics being significantly deteriorated.
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a broad pass bandwidth that minimizes generation of parallel capacitances at the interstage junctions of longitudinally connected surface acoustic wave elements so that the impedance of each element is hardly capacitive.
According to a preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate, a plurality of longitudinally-coupled resonator-type surface acoustic wave elements having a plurality of interdigital transducers (IDTs) disposed on the piezoelectric substrate in a direction in which a surface acoustic wave propagates, electrode pads defining input/output terminals of the longitudinally-coupled resonator-type surface acoustic wave elements, and wiring patterns for electrically connecting the plurality of longitudinally-coupled resonator-type surface acoustic wave elements. In this filter, at least one pair of the plurality of longitudinally-coupled resonator-type surface acoustic wave elements is longitudinally connected to each other via the wiring patterns, and at least one of the electrode pads is arranged between the longitudinally connected surface acoustic wave elements.
In this arrangement, since at least one of the electrode pads defining input/output terminals is arranged between the longitudinally connected surface acoustic wave elements, parallel capacitances generated at the interstage junction of the surface acoustic wave elements are minimized and eliminated.
In addition, according to another preferred embodiment of the present invention, a surface acoustic wave filter includes the structure of the filter according to the preferred embodiment described in the preceding paragraph. In this filter, at least one of the longitudinally-coupled resonator-type surface acoustic wave elements is one of a balanced-input type and a balanced-output type so as to achieve a balance-unbalance conversion function.
When applying the structure of the filter of the first preferred embodiment to the filter having the balance-unbalance conversion function according to the other preferred embodiment, parallel capacitances generated at the interstage junction are minimized, thereby improving balancing in the balance-unbalance conversion, which is an additional advantage that is achieved in this structure.
Furthermore, according to another preferred embodiment of the present invention, a communication apparatus includes the surface acoustic wave filter according to one of the above-described preferred embodiments.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention.